1. Field of the Invention
The present invention relates to insulation test equipment for testing high voltage electrical equipment at the rated voltage level while limiting the maximum output current to a safe level.
2. Description of the Prior Art
Motors and generators are used in many industries and institutions. Offshore drilling rigs typically have one or more generators to meet their power requirements, as well as motors to perform various tasks on the rigs. Hospitals often have backup generators for use in the event of a power outage. Such motors and generators operate at relatively high power and voltage levels. For example, the voltage ratings of the motors are typically 250 V, 500 V, 1 kV and 5 kV. This electrical equipment is expensive and must be properly maintained, inspected and tested periodically to assure safe and reliable operation.
A common cause of malfunctioning of electrical motors and generators is inadequate insulation of the wiring. Such inadequate insulation may be due, for example, to manufacturing defects of the wiring, shorts introduced during manufacture of the equipment, or breakdown of the insulation. Generally, breakdown of the insulation typically occurs when the equipment is idle. Common causes of insulation breakdown, in addition to thermal aging, include the presence of moisture, oil, grease, chemical fumes and airborne contaminants. If the equipment is not tested each time before it is run, the testing may be useless because, if the insulation breakdown is serious enough, the equipment may fail and even explode on start-up.
The insulation resistance can be measured directly by opening the motor or generator, applying a DC voltage to the wires, and measuring the current which flows to ground. This manual method of insulation monitoring is time consuming and, therefore, is not performed as often as it should be. The inevitable result is that insulation breakdown goes undetected, and is often not discovered until a motor or generator fails when it is started at great expense to its owner.
Automatic insulation resistance testers have been built and used for testing electrical equipment having voltage ratings up to 500 volts. U.S. Pat. No. 5,155,441 discloses an automatic insulation tester for periodically testing a plurality of motors or generators. A related patent, U.S. Pat. No. 5,172,289, discloses an automatic insulation tester using a ground interrupter to interrupt the normal ground circuit of the electrical equipment before it is tested for insulation breakdown. The operation of the insulation testers in these two patents is similar. When the electrical equipment is not operating, the insulation tester automatically checks the equipment and disables the starting mechanism if the equipment is not safe to be operated. The insulation tester uses relay logic to disable itself while the motor or generator is operating. Typically, a potential transformer is connected to two of the input lines to provide power to an insulation tester. A low-voltage power supply converts the sinusoidal line voltages to a low DC voltage, which is provided to a "high" voltage switching power supply. The high voltage switching power supply typically uses a high impedance isolator circuit and a four to one voltage amplifying transformer to develop the rated voltage level of about 500 volts DC.
The 500 volts DC, which is the rated voltage level of the equipment to be tested, is applied between the windings and ground to determine the leakage current through the insulation of the windings on a three phase motor. The leakage current is used to derive the insulation resistance, which is displayed to the operator during the test. The insulation testers further include latch circuitry to disable the motor starter if the impedance or resistance of the insulation falls below a predetermined low level. Once the insulation resistance test fails, the equipment may only be started after increasing the insulation resistance and resetting the test equipment. Simply resetting the test equipment will not allow operation. Thus, the motor or generator is tested at its rated voltage level of 500 volts while idle to assure safe operation before starting.
Many problems have been encountered when attempting to apply the same principles in developing insulation test equipment for motors or generators having voltage ratings above 500 volts. It is possible to provide a high impedance oscillator to drive a transformer having ten times the number of windings to develop the voltage level necessary to test electrical equipment having ten times the rated voltage level. Or, several transformer stages could be serially linked to develop the necessary voltage levels. Even though such test equipment could be designed, the test equipment would also be capable of driving a significant amount of current, which could easily injure or even kill a human operator upon accidental contact. Such equipment is simply too dangerous to be used by or near humans.
As a result, the automatic insulation test equipment available today for measuring insulation resistance tests the high voltage electrical equipment at a much lower voltage level than the rated voltage level, to assure safety. This technique is not desirable, since it is based upon the incorrect assumption that there is a linear relationship between the voltage applied and the current measured through the insulation. This assumption is inaccurate and often provides faulty data, which could be hazardous if a fault condition truly exists. It is known that tests using voltage levels substantially below the rated voltage levels tend to inspire confidence which may not be justified. It is believed that a test which does not stress the insulation at or greater than the service potential is a waste of time and money. If there are no problems, the lower or higher test voltages may reveal nothing. However, the higher test voltages reveal latent weaknesses that the lower test voltages will miss. Nonetheless, lower voltage test equipment is commonly used to test equipment having higher rated voltage levels, since no other method has been practical.
Therefore, it is desirable to provide a safe and reliable means for automatically measuring insulation resistance of high voltage electrical motors and generators at or above the rated voltage level.